1. Field of the Invention
The present invention relates to substrate via technology.
2. Background Art
Electrical impedance, or simply impedance, describes a measure of opposition to alternating current (AC). When an AC signal is transmitted through a conductor, such as a metal trace, the conductor provides some resistance to the AC signal in the form of electrical impedance. The path of the signal may include further features beyond the conductor, such as further conductor including further traces, vias (electrical connections through layers of circuit boards), and other circuit elements or features that each have corresponding impedances. When the signal conducts through the conductor and encounters a subsequent conductor, the signal may be distorted in some manner. If the impedance of the subsequent conductor matches the impedance of the prior conductor, the signal may continue through the subsequent conductor in an undistorted manner. If the impedance of the subsequent conductor does not match the impedance of the prior connector, a portion of the signal may continue through the subsequent conductor, and a portion of the signal may be reflected back down the prior connector. As such, the mismatched impedances cause a distortion to the signal, which is undesired.
Insertion loss (IL) and return loss (RL) are signal parameters used to indicate the magnitude of signal distortions in the event of mismatched impedances. Insertion loss is the loss of signal power resulting from the presence of a discontinuity in a signal path. Return loss (or reflection loss) is the loss of signal power resulting from the reflection caused at a discontinuity in a signal path.
Integrated circuit (IC) chips or dies from semiconductor wafers are typically interfaced with other circuits using a package that can be attached to a printed circuit board (PCB). Many types of IC packages exist. One such type of IC die package is a ball grid array (BGA) package. BGA packages provide for smaller footprints than many other package solutions available today. In a BGA package, one or more dies are mounted to a package substrate. An array of solder ball pads is located on a bottom external surface of the package substrate. Solder balls are attached to the solder ball pads. The solder balls may be reflowed to attach the package to the PCB. Further examples types of IC packages include quad flat package (QFP) (having leads extending from all four lateral sides), quad flat no lead (QFN) packages, etc.
A die is frequently mounted to a package substrate using solder balls, studs, or other structures that electrically couple signal pads of the die to land pads on the package structure. Such solder balls, studs, or other structures form signal interconnects. As such, a signal transmitted from an output driver of a die in an IC package passes through the interconnect between the die and the package substrate, one or more traces and vias of the package substrate, and an interconnect between the IC package and a circuit board. The voltage and current that make up the signal encounter each interconnect, trace, and via as an electrical impedance. If the impedances of the features through which the signal pass are the same, the signal may continue undistorted. If, however, the impedances of the features do not match, a portion of the signal will reflect from the point at which the mismatch occurs, and a portion of the signal will continue on in a distorted fashion.
In an example BGA package, a metal trace segment may make up a longer portion of the overall signal path, and may have a control impedance typically targeted in a range of 90 to 110 Ohms (differential). A via structure and BGA sections (the BGA pads and associated metal traces) of the BGA package make up the smaller segment of the overall signal path, and may typically have a lower impedance around 60 to 70 ohms differential. This change in the impedance profile between the metal trace and the via and/or the BGA section is an impedance discontinuity that can cause significant deteriorating effects on insertion loss, return loss, and the signal quality on the receiver end.
Previous techniques to match impedances in BGA packages in the industry include using “cut-outs” around solder ball pads to reduce capacitance, and therefore increase the impedance of the BGA section of a BGA package. However, the limits of this type of reduction may be reached before the desired target impedance is attained (i.e., the impedance does not increase enough to reach the desired value). In another previous technique, a planar two-dimensional trace is configured to create coiling to modify an electrical impedance in the signal path. However, a problem with this technique is that the additional trace that is used degrades insertion loss (e.g., due to conductor, surface roughness, and dielectric losses) and occupies a significant amount of space and substrate real-estate.